Use registryHTTPResponseToError on /tags/list failure

We should have been doing that from the start, in order to
get server-reported errors correctly.

NOTE: This can break existing clients, notably (skopeo inspect)
which greps the error string for 401 and 403.  See the corresponding
skopeo PR for documentation of what we do about that.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

docker/docker_image.go

@@ -77,8 +77,8 @@ func GetRepositoryTags(ctx context.Context, sys *types.SystemContext, ref types.
 			return nil, err
 		}
 		defer res.Body.Close()
-		if err := httpResponseToError(res, "fetching tags list"); err != nil {
-			return nil, err
+		if res.StatusCode != http.StatusOK {
+			return nil, fmt.Errorf("fetching tags list: %w", registryHTTPResponseToError(res))
 		}
 
 		var tagsHolder struct {

docker/errors_test.go

@@ -110,6 +110,7 @@ func TestRegistryHTTPResponseToError(t *testing.T) {
 				var e errcode.Error
 				ok := errors.As(err, &e)
 				require.True(t, ok)
+				// Note: (skopeo inspect) is checking for this errcode.Error value
 				assert.Equal(t, errcode.Error{
 					Code:    errcode.ErrorCodeUnknown, // The NOT_FOUND value is not defined, and turns into Unknown
 					Message: "404 page not found",